The handwringing over this strikes me as extreme. Mozilla already ships with the ability for users to enable closed-source, proprietary DRM schemes to view web content -- the Flash and Silverlight plugins. Moving from the current regime of Flash/Silverlight to the EME is a move towards openness, not away from it -- the DRM scheme is still closed, but the content itself is more open and standard than before, as you're…
In concept, I think it's not too much different from server-side website code being mostly closed source. The only difference is that this component happens to be running on your computer.
Also there is prior form for this in similarly open systems, e.g. tainted modules in the Linux kernel, which seem to have turned out okay.